But these classes are not performance critical. Not only that, but sealing classes only increases chances of negligibly increasing performance and should only be used as last resort in that case.
While they do not /require/ extension, that does not mean that there is no case when it is convenient to do so.

IMHO limiting code extensibility by making micro optimizations is bad architectural decision. After all "The First Rule of Program Optimization: Don't do it".

There and without it many classes and interfaces for extension: ITransformer and
TransformerBase, ITranslator and TranslatorWithNativeMinificationBase,
IPostProcessor and PostProcessorBase, and IMinifier.

Well that definitely makes it harder to work around bugs.
Whatever. At least accept my pull request where I prevent throwing exceptions when bundle does not have any resources.
I hope release cycle is not too long...

I assemble bundles from folders based on controller and action. Sometimes folders are empty, therefore empty bundles are formed.
I could check if bundle is not empty before calling Scripts.Render(), but Bundle.EnumerateFiles() is bothersome to call, not to mention performance penalty since it always creates a new list.
From logical point of view rendering empty bundle should return empty result instead of throwing exceptions. The is nothing wrong to have an empty bundle.

IBundleTransform

IBundleTransform was my first choice, but it doesn't really work well with Handlebars in debug mode. Implementing IPostProcessor was not as complicated.